GNOME Flashback (previously called GNOME fallback mode) is a shell for GNOME 3. The desktop layout and the underlying technology is similar to GNOME 2. It doesn't use 3D acceleration at all, so it's generally faster and less CPU intensive than GNOME Shell with llvmpipe.

Using GNOME Panel with a standalone window manager

The GNOME Flashback experience is primarily provided by GNOME Panel. The other components are only required for the Flashback session itself. If you would like to use GNOME Panel without the Flashback session you can just install the gnome-panelAUR package and autostart the panel with a standalone window manager such as Openbox.

For example: if you want to use GNOME Panel with Openbox add gnome-panel & to the ~/.config/openbox/autostart file.

See your window manager's wiki page as well as the Autostarting page for further instruction.

Troubleshooting

Mouse cursor is not visible

When used with Gnome 3.10 the mouse cursor is not visible. As a workaround, in the dconf-editor expand: org -> gnome -> settings-daemon -> plugins and click on 'cursor.' Uncheck the box labelled active.